In the prior art, the method of conventional valve actuation for a vehicle engine is well known and its application has more than one hundred years of history. However, due to the additional requirements on engine emission and engine braking, more and more engines need to produce an auxiliary engine valve event, such as an exhaust gas recirculation event or an engine braking event, in addition to the normal engine valve event. The engine brake has gradually become the must-have device for the heavy-duty commercial vehicle engines.
The engine braking technology is also well known. The engine is temporarily converted to a compressor, and in the conversion process the fuel is cut off, the exhaust valve is opened near the end of the compression stroke of the engine piston, thereby allowing the compressed gases (being air during braking) to be released. The energy absorbed by the compressed gas during the compression stroke cannot be returned to the engine piston at the subsequent expansion stroke, but is dissipated by the engine exhaust and cooling systems, which results in an effective engine braking and the slow-down of the vehicle.
There are different types of engine brakes. Typically, an engine braking operation is achieved by adding an auxiliary valve event for engine braking event into the normal engine valve event. Depending on how the auxiliary valve event is generated, an engine brake can be defined as:
1. Type I engine brake: the auxiliary valve event is introduced from a neighboring existing cam in the engine, which generates the so called Jake Brake;
2. Type II engine brake: the auxiliary valve event generates a lost motion type engine brake by altering existing cam profile, for example the integrated rocker arm brake;
3. Type III engine brake: the auxiliary valve event is produced from a dedicated brake cam, which generates a dedicated brake valve event via a dedicated brake rocker arm;
4. Type IV engine brake: the auxiliary valve event is produced by modifying the existing valve lift of the engine, which normally generates a bleeder type engine brake; and
5. Type V engine brake: the auxiliary valve event is produced by using a dedicated valve train to generate a dedicated valve (the fifth valve) engine brake.
An example of engine brake devices in the prior art is disclosed by Cummins in U.S. Pat. No. 3,220,392 in 1962. The engine brake system based on the patent has enjoyed a great commercial success. However, this engine brake system is a bolt-on accessory that fits above the engine. In order to mount the brake system, a spacer needs to be positioned between the cylinder and the valve cover. This arrangement may additionally increase height, weight, and cost to the engine.
The above engine brake system transmits a mechanical input to the exhaust valves to be opened through a hydraulic circuit. The hydraulic circuit generally includes a master piston reciprocating in a master piston hole, and the reciprocating motion comes from a mechanical input of the engine, such as the rocking of the injector rocker arm. Through hydraulic fluid, the motion of the master piston is transmitted to a slave piston located in the hydraulic circuit, thereby causing the slave piston to reciprocate in the slave piston hole. The slave piston acts, directly or indirectly, on the exhaust valves, thereby generating the valve event for the engine braking operation.
The conventional engine brake with hydraulic actuation has another drawback, i.e. the contractibility or deformation of the hydraulic system, which is relevant to the flexibility of the fluid. High flexibility greatly reduces the braking valve lift, the reduction of the braking valve lift leads to the increase of the braking load, and in turn the increased braking load further causes much higher flexibility, thereby forming a vicious circle. In addition, the braking valve lift reduction caused by the hydraulic deformation increases with the increase of the engine speed, which is against the braking valve lift trend required by the engine braking performance. In order to reduce the hydraulic flexibility, a hydraulic piston with a large diameter must be used, which increases the volume and weight. And, it will take a long time for the oil flow to drive such a large diameter piston to extend or retract, which increases the inertia and response time of the engine brake system.
One of the earliest engine brake systems integrated in the engine within the existing parts is disclosed in U.S. Pat. No. 3,367,312 by Jonsson in 1968, which is an integrated compression release engine brake system. The brake system is a lost motion type engine brake that needs to modify the conventional cam of the engine. In addition to enlarge the conventional cam lobe for power operation, brake cam lobes for engine braking are added on the same cam. The rocker arm of the brake system is installed on an eccentric cylinder surface of the rocker arm shaft. The rocking center position of the rocker arm is changed by rotating the rocker arm shaft, thereby causing or eliminating a gap for the “lost motion” between the cam and the engine valve. When the gap is formed, the motion from the braking cam lobes is lost, and the engine only generates power operation. When the gap is eliminated, the motion from all the cam lobes (the enlarged conventional cam lobe and the braking cam lobes) is transmitted to the engine valve, thereby producing the auxiliary valve event for the engine braking operation.
In Jonsson's brake system, when rotating an eccentric rocker arm shaft and changing the rocking center positions of all rocker arms, many valve spring forces on the rocker arm must be overcame, which results in a large hydraulic actuation system. Another drawback of the Jonsson's brake system comes from the enlarged conventional valve lift profile during the engine braking caused by the enlarged conventional cam lobe, which reduces the braking power and increases the injector tip temperature.
U.S. Pat. No. 5,335,636 (in 1994) discloses another integrated rocker brake system. The brake system also needs to modify the conventional cam of the engine. In addition to enlarge the conventional cam lobe for the power operation, a brake shoulder for the engine braking is added to the same cam. The brake shoulder is a cam lobe with a fixed (constant) height and can only be used for a bleeder type engine braking, and can not be used for the compression release engine braking. In addition, the rocker arm of the brake system is installed on an eccentric bushing, and the eccentric bushing is installed on the rocker arm shaft. By rotating the eccentric bushing and changing the rocking center position of the rocker arm, a gap for the “lost motion” is formed or eliminated between the cam and the engine valves. When the gap is formed, the motion from the braking shoulder on the cam is lost, and the engine only generates the power operation. When the gap is eliminated, the motion from all the cam lobes (the enlarged conventional cam lobe and the braking shoulder) is transmitted to the engine valve, thereby producing the auxiliary valve event for the engine braking operation. Also, the rocker arm of the brake system acts on a valve bridge and opens two valves simultaneously for the engine braking operation.
The above integrated rocker arm brake system still needs to enlarge the conventional cam lobe, which leads to an enlarged conventional valve lift during engine braking, a lower braking power and a higher injector tip temperature. In addition, the integrated rocker arm brake system can only be used for a bleeder type engine braking, and can not be used for a compression release type engine braking. The bleeder type engine braking has much lower braking performance than the compression release braking. Also, opening two valves for engine braking doubles the braking load on the entire valve actuation mechanism, which results in more wear and worse reliability and durability.
U.S. Pat. No. 5,647,319 (in 1997) discloses another integrated rocker brake system utilizing an eccentric bushing. The brake system is also a bleeder type engine brake, wherein the braking valve lift has a constant height, however the brake system has two different braking valve lifts. The smaller braking valve lift is used for low engine speeds (below 2000 rpm) and the higher braking valve lift is used for high engine speeds (above 2000 rpm). In addition, in all integrated rocker arm brake systems, the engine's ignition operation and braking operation share the same cam, and the existing conventional cam lobe needs to be modified, which may lead to an mutual influence between the ignition operation and the braking operation, a lower braking power, a higher injector tip temperature, an increased wear of valve train components, and a reduced engine reliability and durability.